Michael Moore is the most successful documentary film- maker in the world. He has tackled contentious subjects, including factory closures (1989's Roger & Me), gun control (Bowling for Columbine, Oscar for best documentary) and American health care (Sicko).

Fahrenheit 9/11 in 2004, his take on then United States president's George W Bush's war on terror, made more than US$222 million at the box office, more than any other documentary, and all of them have Moore's trademark humour.

It means Moore can pick and choose his subjects, and none come bigger and more foreboding than his latest - Capitalism: A Love Story. And Moore's verdict on the world's most popular and pervasive economic system and ideology?

"I think it's an evil system that's capable of the worst of humanity. It's so odd that we are even discussing really a 16th- century philosophy, when we live in such a different world in the 21st century.

"We need to come up with something that speaks to the needs of how to live in this new world."

Moore is telling me this as he relaxes, munching grapes, at a hotel in Florida. It's not that Moore is by the pool, soaking up the sun like some successful venture capitalist. Such is his drive to promote his new feature that he is happy to start the interview at 12.45am after he has been out promoting it in Miami.

"I was driving down the street past a movie theatre, and on the marquee, there it is in big letters: Capitalism. They've got six movie theatres, so they didn't have room to put the whole title. So what's showing tonight is Zombieland, Whip It and Capitalism," he says with glee.

Moore's latest film is very much in the style of his previous documentaries. The tone's never sombre, even if the message is. It's part autobiographical. There's the often funny use of old news footage.

Some people he interviews are willing participants. Others try to throw him out when he arrives unannounced.

To back Moore's thesis, he cites Wall Street and corporate America bailouts since the recession, home foreclosures, the wide discrepancy in wealth, a secret Citibank memo that happily declares that the US is not a democracy but a "plutonomy" - an economy largely for the benefit of the rich. Capitalism's supporters and critics speak, we're shown workers' co-ops and sacked unpaid workers barricading themselves in factories.

Then there are the bizarre examples. The "dead peasants" life insurance, where large American companies take out life insurance on their employees without their knowledge, and vulture- like speculators selling off foreclosed homes. Much of the footage in Capitalism: A Love Story is very recent, as late as the middle of this year, but Moore says he began to think about making it three years ago and was six months into working on it when the economic crash happened.

"I was tired of making single-issue films when I was really talking about the economic system that I have believed since I was a teenager was unjust and undemocratic.

"The crash happened right while we were making the film about the crash that was probably going to happen."

Moore says he did fear that capitalism was too much of a behemoth to be pinned down in a documentary, but he was determined to challenge himself.

"After 20 years of doing this, I can either pick topics and phone it in or I can choose things to do that are impossible for me to do.

"To make it even more difficult on myself, let's give it a title that absolutely no-one would think of taking a date to. The movie's going to have to be really good if you're going to put the word 'capitalism' in the title. Nobody's going to be thinking about, after working hard all week, 'Let's go to that Capitalism movie tonight to relax'."

Some have criticised the film for not offering an alternative - other than democracy. But Moore is blunt - he doesn't believe the best alternative has been worked out yet.

"Basic socialism has democratic principles. It's about sharing the pie and that everyone is supposed to have a seat at the table.

"But when it has been tried, sometimes when it hasn't worked it's been because it seeks to deny the individual. We're not going to come up with the next great invention, or the thing that's going to make the world a better place, if we don't allow the individual spirit."

Moore says only one thing surprised him while making the documentary.

"For the first time, Republicans and conservatives were inviting me into their homes. I think that they realised now that they had been lied to about me by the [Rupert] Murdoch channel [Fox] and by right-wing hate radio.

"I'd been right too many times in trying to warn them, starting 20 years ago about General Motors [in Roger & Me] and then telling them that we would find no weapons of mass destruction in Iraq and we were being lied to about that, to trying to ignite this health-care debate two years ago.

"So I have a track record with people who may not agree with me politically on a number of things, but know that I am going to be there to fight for them and for people like them who get the short end of the stick."

But Moore also has his critics. He never reads reviews of his films, he says. He is aware of around a dozen anti-Moore documentaries, although not 2007's Manufacturing Dissent made by left-wing Canadian film-makers, which screened in New Zealand. It claimed Moore used "misleading tactics". It's the only time in the interview that he loses his affability.

"Anybody who says that my facts are wrong is libelling me. My facts are never wrong. My facts are always right. I have such an incredible research team and I have a battery of fact-checkers and lawyers who go through my movies before they're released. In the last three films I've taken to offering $10,000 to anyone who can find a fact that's wrong.

"Now, the opinions in the films are mine. I think they're right because they're my opinions, but I may not be right. I might be wrong. It might be a good idea to have a gun in every home. I don't think so."

Of his success, Moore says there is only one drawback: "The hate speech against me from the right wing encourages unstable people to think about hurting me.

"Everything else about it has been positive. I've been blessed to be able to have my films distributed widely in theatres in shopping malls, not just in art-house cinemas. People say to me, 'It must be hard to walk into these corporations now, because they know you'.

"Actually, it's just the opposite. It's really easy, because the people who work at these companies who want to be whistle blowers send me documents. I have so much stuff sent to me that I could make a film a week."